1. Field of the Invention
The present invention relates to a thin film device including a compound having ionic bonding (called “ionic crystal” from now on) and its fabrication method, and more particularly to a thin film device and its fabrication method preferably applicable to electronic devices utilizing an ionic crystal thin film as a functional film, such as a high brightness blue light emitting device (semiconductor laser), metal insulator semiconductor field effect transistor (MISFET), high electron mobility transistor (HEMT) and thin film capacitor.
2. Description of the Related Art
Nitride, oxide and sulfide each exhibit a variety of physical properties. Although they are valuable functional materials as polycrystalline, their single crystals can become more effective or exhibit new characteristics. When applying these materials to thin film devices, forming their single crystal thin films can implement high performance, high function devices unachievable by polycrystalline thin films.
For example, consider devices using nitride thin films such as a high brightness blue light emitting device using GaN thin films, a MISFET using AlN/GaN thin films and a HEMT using AlGaN/GaN thin films. Although a variety of such devices have been proposed and implemented, unless the nitride thin films consist of single crystal thin films without little lattice defects or grain boundary, their carrier mobility can be reduced, or the light emitting efficiency of their light emitting layers or their lifetime can be reduced.
As for the single crystal thin films, it is common that they are epitaxially grown using a single crystal substrate. For example, there have been reported that GaN-based single crystal thin films are formed on a single crystal sapphire substrate by an MOCVD (Metal Organic Chemical Vapor Deposition) method or by a gas source MBE method (Molecular Beam Epitaxial Method), or on a SiC substrate by a low pressure metal organic vapor phase epitaxy (see, Kiyoteru Yoshida “Electronic devices using GaN” pp. 787–792 and Kuramata et al., “Continuous-Wave Operation InGaN Laser Diodes On SiC Substrates,” pp. 797–800, OYOBUTURI Vol. 68, No. 7, published by The Japan Society of Applied Physics, Jul. 10, 1999). As for the oxide-based single crystal thin films, their epitaxial thin films are formed on a substrate composed of single crystal strontium titanate (SrTiO3:STO), single crystal lanthanum aluminate (LaAlO3:LAO) or single crystal sapphire substrate by a sputtering or PLD method (Pulsed Laser Deposition).
However, since the sapphire substrate, SiC substrate, single crystal STO substrate and single crystal LAO substrate are expensive, it is preferable that the thin films be formed on a general purpose Si substrate. In addition, from the viewpoint of a merger with Si devices, it is preferable that the functional thin films be epitaxially grown on a Si substrate. However, it is difficult to epitaxially grow an ionic bonding thin film directly on a Si single crystal substrate. The reason for this is considered that since the silicon is a covalent crystal, a material with a lattice constant differ from that of Si by a few percent cannot grow coherently (pseudomorphically) on a substrate, thereby leaving lattice defects.
As a method of forming a thin film on a Si single crystal substrate, a method of inserting a buffer layer is known.
A commonly used method is to form a metal oxide of CeO2, Y2O3, ZrO2 and the like, which are more susceptible to oxidation than Si, thereby preventing non-crystalline (amorphous) SiO2 from being generated. However, it is unavoidable that the Si surface undergoes oxidation, which offers a problem of degrading the film characteristic of the buffer layer formed on the SiO2. In addition, a buffer layer using TiN or TaN also has a problem of forming SiNx.
To cope with this, a method using a metal sulfide thin film as the buffer layer is proposed (see, Japanese Patent Application Laid-open No.2002-003297). According to the relevant document, the formation Gibbs' energy of forming the Si sulfide is rather small. Thus, when its lattice constant is close to that of Si, it is possible to epitaxially grow the sulfide without forming an amorphous layer on an interface between the buffer layer and Si. The Japanese Patent Application Laid-open No.2002-003297 discloses that the elements such as Al, Ba, Be, Ca, Ce, In, La, Li, Mg, Mn, Mo, Na, Sr, Ta and Zr have the formation Gibbs' energy greater than Si in forming the sulfide, and that using a sulfide composed of one of them or a combination of them can suppress the Si interface reaction, and proposes an oxide thin film device and its fabrication method using the metal sulfide layer as the buffer layer.
Furthermore, CdS and ZnS also have the formation Gibbs' energy greater than that of Si in forming the sulfide.
However, a thin film device including epitaxial thin films for a laser diode or LED formed on a Si substrate, and its fabrication method have not been implemented.